Gross Primary Production: aquatic ecosystems = ?

I am looking for a value for Gross Primary Productivity in aquatic
ecosystems. Though NET Primary Productivity estimates are common (~2000 g
m-2 yr-1 for swamp and marsh) (Begon, Harper and Townsend, 1990), I am
seeking a value for total photosynthetically fixed energy, without
subtracting for respiration. In my case, I measured not increase in
biomass
over a year, but increase in dissolved oxygen over a matter of hours. Any
suggestions out there?
Bruce Ratcliffe, Fresno